Various types or forms of fluidic conduits, such as tubes, columns and linear flow cells are used in analytical instrumentation for transporting and/or processing fluids and samples. The conduits in such systems often include tubing assemblies where different conduits are coupled together to form fluidic connections between them.
Fluidic conduits employed in chemical analysis instruments, such as, for example, chromatography (LC), high performance liquid chromatography (HPLC), ultra performance liquid chromatography (UPLC®), capillary electrophoresis (CE) and capillary electro-chromatography (CEC), can have demanding performance criteria. For example, a liquid chromatography system, such as a system designed for ultra performance liquid chromatography (UPLC®), can operate at pressure that may exceed 18,000 psi.
Problems associated with forming fluidic connections between tubing assemblies are particularly prominent for those required to withstand high-pressure operation, for example, pressures in the range 10,000 to 18,000 pounds per square inch (psi), and for systems where the tubing dimensions are small, for example where the area of the tubing endface is large relative to the area of the tubing opening and/or where the thickness of the tubing wall is large relative to the tubing inner diameter.
A primary concern in high pressure fluidic systems is leaks where two different tubes are coupled. In typical couplings, the seal is formed along the side of the tube. For example, many couplings use an annular sealing element such as a ferrule that has a conical outer surface. To form a fluid-tight coupling, a tube having the annular sealing element displaced away from the endface is inserted into a receptacle of a coupling body. The receptacle is defined by a cylindrical bore that transitions to a conical bore and then to a smaller diameter cylindrical bore. A fluid channel extends from the surface at the bottom of the smaller diameter cylindrical bore into the coupling body. The cone angle of the conical bore is greater than the cone angle of the annular sealing element resulting in a seal along the circumferential contact between the annular sealing element and the conical surface of the conical bore. Additional force applied by a compression screw after achieving initial contact between the annular sealing element and conical bore surface results in a contact seal between the annular sealing element and the outer surface of the tube. If the endface of the tube is not in contact with the bottom of the cylindrical bore, the region between the outer surface of the tube and the side wall of the smaller cylindrical bore below the circumferential contact seal represents a dead volume, e.g., a volume that is unswept or only partially swept by the flowing fluid. Such dead volumes can result, among other things, in sample entrapment and carry-over. However, such circumferential seals allow for greater control of the sealing process which can be critical when elements such as silica capillary tubes are used.
An analytical technique that relies upon accurate flow volumes, accurate sample concentrations and/or reproducibility will be adversely affected by dead volume. In chromatography adverse effects such as carryover and peak tailing can result from the use of conventional couplings used to achieve fluid-tight seals. The introduction of dead volume can be especially detrimental for tubing of smaller dimensions as such dead volumes can become proportionally larger as the tubing dimensions decrease.